WHITE SHRIMP FROM THE GULF OF MEXICO 



117 



ventrad. At this point the structure articulates 

 with the pleopod coxopodite. The mesial V, 

 which also points ventrad, constitutes the support 

 for the inner articulation of the pleopod coxo- 

 podite. The mesial V is a sternal element. The 

 dorsal or free ends of the lateral and mesial arms 

 of the V's are joined. The resulting ventral skel- 

 etal structure represents a set of combined tri- 

 angles, if the coxopodite is included as a struc- 

 tural member, which can be described as a kind of 

 Warren truss. The structural triangles provide 

 support for the articular foramen to which the 

 pleopod is attached. The ventral skeletal struc- 

 ture pertaining to the support of the pleopod is 

 lighter in construction in Penaeus than in Asta- 

 cus or Cambarus, although the pleopods of 

 Penaeus are comparatively much larger and more 

 powerful. 



A further structural difference of note between 

 the ventral skeletal pleopod support in Penaeus 

 and that of the Astacura and the Braehyura is the 

 position of the appendage relative to the ventral 

 abdomen. In the Astacura particularly, the artic- 

 ular foramen of the pleopod is fully ventral, the 

 ventral sternum and the so-called ventrolateral 

 pleural plates being flat and lying horizontally. 

 In Penaeus by contrast the sternum is convex and 

 the lateral plates heretofore considered pleural 

 are nearly vertical, placing the pleopod in a ven- 

 trolateral position exposed to the water in which 

 the organ functions. Although Berkeley does not 

 consider the subject, her illustrations indicate that 

 the situation in Pandalus is similar to that in 

 Penaeus. 



The coxopodite (fig. 65) of the pleopod is a 

 narrow, incomplete band or ring which articu- 

 lates with the ventral skeletal elements dorsally 

 and the basipodite ventrally. To the coxopodite 

 are inserted muscles having their origins on the 

 tergal plates of the abdominal skeleton and in the 

 basipodite. The basipodite (fig. 65) is a broad 

 and elongate structure, shaped like an inverted 

 heart. The form of its cross section through the 

 broad, distal portion is streamlined to reduce its 

 resistance to the water during the recovery stroke. 

 The basipodite articulates with the coxopodite 

 proximally and with the endopodite and exopo- 

 dite distally. By virtue of the arrangement of 

 points of articulation and of its extensive muscu- 

 lature, the basipodite substantially reinforces the 

 length and power of the propulsive stroke of the 

 pleopod. 



Distal to the basipodite, and visible only on 

 the mesial side of the pleopod, lies a small, heav- 

 ily sclerotized structure (fig. 68), the exopodite 

 articular element, which articulates with the basi- 

 podite and the exopodite. Until more informa- 

 tion is available, this structure will be considered 

 an exopodite component. In Penaeus, several 

 muscles insert in the structure. No trace of this 

 appendage segment appears in the modified pleo- 

 pods of Astacus, Cambarus, or CaHinectes. 

 Berkeley makes no reference to the structure in 

 Pandalus. 



The endopodite (fig. 68) articulates with the 

 basipodite by a somewhat constricted area on the 

 mesial side of the pleopod. Functionally the 

 endopodite is simple, accompanying the exopodite 

 in its movements. The exopodite (figs. 65 to 69) 

 is a large, broad, oar-shaped organ, convex ante- 

 riorly and concave posteriorly. Like the endopo- 

 dite. it is comprised of a long series of lightly 

 sclerotized rings alternating with bands of thin 

 cuticle. The skeleton of the exopodite is arranged 

 thereby to bend when a stream of water strikes it 

 from the anterior and to remain straight when 

 water strikes it from the posterior. The exopodite 

 functions like a curved, steel carpenter's rule which 

 may be rolled on a spool in one direction, but which 

 presents resistance to bending in the opposite di- 

 rection. The functional surface of the endopodite 

 and exopodite oar blades is greatly increased by 

 long, plumose setae embedded in the margins. The 

 pleopod endopodite and exopodite have been 

 called flagella in Astacus and other reptant deca- 

 pods in which the organs are extremely reduced 

 by comparison with those of Penaeus and Panda- 

 lus. The term will not be used here, since the 

 endopodites and exopodites of Penaeus setiferus 

 are not flagella. 



MUSCLE ELEMENTS 



The third pleopod of Penaeus setiferus is op- 

 erated by 17 or more muscles. The number of 

 discrete muscles intrinsic to the pleopod endo- 

 podite in Penaeus is not clear, although the organ 

 may contain two or more muscles. Schmidt in his 

 account of Astacus describes 9 muscles for the 

 third pleopod, Cochran lists 6 for the first pleopod 

 of Callinectes, Hart (1952) describes 9 for the 

 third pleopod of Cambarus longulus longulus, 

 and Berkeley finds 11 in Pandalus. 



